Birefringent filter-based color generation scheme for a passive matrix display device

ABSTRACT

The present invention provides a full color liquid crystal display of passive matrix design that includes a polarizer, an analyzer having a transmission axis, and at least one light generation stage positioned between the polarizer and analyzer for transmitting light of a desired wavelength to the analyzer. This light generation stage includes a first retarder stack that rotates desired wavelengths of light to a particular polarization state, a second retarder stack inverted and rotated by 90 degrees with respect to said first retarder stack, and a passive matrix addressed optical element positioned between the first and second retarder stacks. The retardation and orientation of each of the first retarder stack, second retarder stack, and passive matrix addressed optical element are optimized so as to provide a color generation stage that places desired wavelengths of light substantially along the transmission axis of the analyzer.

BACKGROUND ART

The present invention generally relates to full color liquid crystaldisplays generated by the use of birefringent filters. Moreparticularly, the present invention relates to full color liquid crystaldisplays using polarization interference filters in a passive matrixaddressing scheme.

Traditionally color generation in liquid crystal displays (LCD) isaccomplished by the patterning of color filters onto individual pixels.However, the use of color filters results in both a loss of resolution,due to subpixeling, and lower light transmission, due to the absorptivenature of the filters. Other methods of generating color include fieldsequential color in which the actual color is generated by RGB lightemitting diodes (LED) and the liquid crystal (LC) pixel is driven so asto either block or transmit the colored light. This typically requiresthe LC to be driven at a very high frame rate which in turn calls for avery fast switching LC material and mode. Such materials and modes aredifficult to find.

Another method of color generation employs polarization interferencefilters (PIF). Such filters work by introducing a phase shift betweentwo orthogonally polarized field components by either a static ordynamic optical element such as a uniaxial retarder or electricallydriven LC cell. Color is generated by the interference of these twocomponents with an analyzer, and color switching is accomplished bychanging the phase shift between the two components by using dynamicoptical elements. Since there is no more absorption than the lossesassociated with polarizer absorption, the PIF can generate color with ahigher luminance than absorptive color filters.

The use of retarders between polarizers to function as filters has beenstudied for several years, starting with the original design formonochromatic imaging by French astronomer Lyot. See I. J. Hodgkinson,Q. H. Wu: Birefringent Thin Films and Polarizing Elements, (WorldScientific Press, Singapore 1997) 1^(st) ed., Chap. 4, p. 52,incorporated herein by reference. The Lyot filter design was improvedupon by the Solc design, which offered similar filtering characteristicsbut higher overall transmission. This design is generally disclosed inIvan Solc, Czechoslovak Journal of Physics, Vol. 10(1), 16-34 (1960),also incorporated herein by reference. The Solc design was thengeneralized by Harris et al., A Optical Network Synthesis UsingBirefringent Crystals, 1. Synthesis of lossless Networks of Equal-LengthCrystals, J. Opt Soc. America (1964) 54(10):1267-1279, incorporatedherein, describing a synthesis procedure for obtaining various filterdesigns.

The two original Solc designs are the “fan” and “folded” type. In thefan design, the wavelengths at which the elements have an even number ofhalf waves of retardation pass through an output polarizer orientedparallel to the input polarizer. In the folded design, the wavelengthsat which the elements have an odd number of half waves of retardationpass through the output polarizer, which is crossed with respect to theinput polarizer. The passband characteristics of the Solc filters aredirectly related to the number of elements used to create the filter,i.e., the larger the number of elements, the sharper the passband.Another crucial element in the transmission function is the size andfrequency of the sidelobes. The Solc filters, as it turns out, are notoptimal in design and give passband sidelobes that can be avoided usingvarious synthesis techniques.

The use of PIFs for the generation of color in a display device wasshown by Sharp et al., U.S. Pat. No. 5,929,946. Starting from a Solcfilter design, the design of a device capable of switching between asingle desired color and black or white, depending upon polarizerorientations, was shown. Given that a folded Solc filter will filter thedesired wavelengths by rotating them by pi/2, a second active opticalelement was added to the filter in order to be able to switch between afiltering and non-filtering action. Particularly, a single, switchablehalf wave plate was placed between the filter and the second half of thefilter was repositioned. Proper repositioning required inverting thesecond half of the filter and rotating it by pi/2, as shown in FIG. 1,and discussed below.

With reference to FIG. 1, a single color generation stage of a displaydevice according to Sharp is designated generally by the numeral 1.Color generation stage 1 includes a first retarder stack 2 (pre filter),a second retarder stack 3 (post filter), which is inverted and rotatedby 90° with respect to first retarder stack 2, and an active matrixaddressed optical element 4 (liquid crystal cell), particularly, a halfwave plate, placed between first and second retarder stacks 2,3. Firstretarder stack 2 is oriented to rotate a desired wave length of light toan angle of pi/4, with respect to the optic axis of the active matrixaddressed optical element 4 (half wave plate). As is generally known,crossed or parallel polarizers (not shown), particularly an inputpolarizer and an output analyzer, are employed to introduce polarizedlight to the color generation stage 1 and to analyze the lighttransmitted through color generation stage 1.

If LC cell 4 is switched so that its net retardation is zero, then thefirst part of the filter, first retarder stack 2, is crossed withrespect to the second part of the filter, second retarder stack 3, asshown in FIG. 1. In this case, the net retardation between the twopolarizers is zero and so the display appears black for crossedpolarizers and white for parallel polarizers. If LC cell 4 is switchedso that it is a half wave plate, then its effect on the filter dependson its orientation in the stack. Given that a folded Solc design rotatesthe desired wavelength by pi/2, then half the filter will rotate it bypi/4. As shown in FIG. 1, light 5 entering the filter from the left willhave the desired wavelength rotated by pi/4 while light entering fromthe right will be rotated by −pi/4 to 3pi/4. Therefore, for light 5 topass through the complete filter, LC cell 4 must take light polarized atpi/4 and rotate it to 3pi/4 degrees. A pi/2 rotation is easilyaccomplished by placing LC cell 4 at pi/4 to the incoming polarization.In this position the LC cell allows the filter to be switched on or offdepending on its own state. For intermediate voltages, LC cell 4 acts asa retarder between 0 and pi/2 and so rotates the incoming linearlypolarized light to some elliptical state. As a result, the desiredwavelength is not completely rotated by the second half of the filterand so suffers absorptive losses at the analyzer. In this manner,intermediate grey scale colors can be created.

The effect of the filter on the rest of the spectra is as follows. Inthe case of a pure folded Solc filter, the undesired wavelengthsentering the filter linearly polarized at 0° are all left unrotated(more so for a multiple plate retarder than a two plate) and areabsorbed at the analyzer, which is crossed with respect to thepolarizer. For the above design, the placement of the LC cell ensuresthat the unwanted spectra remains unaffected. While the desiredwavelength is rotated, by the first part of the filter, to pi/4, therest of the spectra remains at 0°. Thus, the undesired wavelengths areat pi/2 to the LC cell optics axis and the LC cell has no effect onthem. Passing through the second half of the filter, they remainlinearly polarized at 0° and are absorbed by the analyzer.

Therefore, in summary, Sharp design requires the following:

-   -   (1) a polarizer providing linearly polarized white light at a        known orientation;    -   (2) a prefilter to rotate desired wavelengths to an angle of        pi/4 with respect to the optic axis of the LC cell (or other        electro- or magneto-optic modulator) while leaving undesired        wavelengths untouched;    -   (3) an active matrix addressed optical element such as a LC cell        (or other electro- or magneto-optic modulator) that rotates        linearly polarized light by pi/2, i.e., a half wave plate, and        has no effect on light polarized along or orthogonal to its        optic axis; and    -   (4) a postfilter that is a copy of the prefilter except inverted        and rotated by pi/2.

Such a filter design is capable of producing a relatively narrowpassband transmission function (depending on the number of retardersmaking up the pre and post filter) and so can produce a single color orits grey scales from input white light. Such a filter is discussed in S.Saeed, P. J. Bos, Z. Li, SID00 Digest, 830 (2000), incorporated hereinby reference. In order to produce more than one color, i.e., red, green,and blue, a stacked approach is taken to provide a full color liquidcrystal display, as shown in FIGS. 2 and 3. In the stacked approach, acolor generation stage according to FIG. 1 is tuned for each of thethree primary colors (designated as 1B, 1G, and 1R) and then stacked oneon top of the other. In FIG. 2, like parts as compared to FIG. 1 havereceived like numerals, with distinctions made between each stage andits elements by employing the letters B, G, and R to indicate blue,green, and red. Only one polarizer 6 and analyzer 7 is used at the twoends, and, in FIGS. 2 and 3, they have transmission axes that areorthogonal to one another, although parallel configuration mayalternately be practiced. The stacked design works since each stack onlyrotates the wavelengths associated with its color, eg., the blue stackis designed to rotate wavelengths 430 nm to 490 nm while the rest of thespectra is left linearly polarized at 0°. Since the analyzer is placedat the end of the stack, this part of the spectra is able to movethrough the other two stacks, as shown in FIG. 3(B).

In FIG. 3, white polarized light is shown passing through threedifferent stacks of filters, each stack containing three filtersindividually tuned for red (1R), green (1G), and blue (1B) wavelengths.At (A), the filters are all in an optically inactive mode, and theentire spectrum is blocked by the crossed polarizers. At (B), the bluefilter is in an optically active mode, such that the blue spectrum isrotated and the stack ultimately outputs blue light. At (C), all threeof the filters, blue (1B), green (1G), and red (1R), are in an opticallyactive mode, and, thus, all three parts of the spectrum, blue, green,and red, are rotated, and the stack ultimately outputs white light.

Since the entire white light spectra is able to move through the threelayer stack with each layer only affecting selected wavelengths, thestack can produce red, green, blue and white light or any combinationthereof. This however will only be true if the red, green, and bluespectra do not overlap. As mentioned before, the bandwidth of eachfilter depends on the number of elements making up the filter. If onlyfour retarders are used, for example, then the filter's bandwidth willbe quite broad. If this filter was to be placed in the stacked design,it would rotate parts of the undesired spectra along with the desiredspectra. For example, if the red filter is broadband, then it willrotate parts of the green spectra also. As a result, if the stack issetup to be in the white state, i.e., all three layers are rotatingtheir selected wavelengths, then the green would be over rotated, andthe resulting white output would lack some green. Therefore, in a stackdesign, it is important to have each filter's bandpass be as narrow aspossible so as to prevent the overlap that reduces efficiency.

There are a number of orientations that the retarders making up thefilter can have and still provide similar if not identical transferfunctions. Synthesis techniques described by Harris et al., cited above,and Buhrer, Appl. Opt. 33, 2249-2254 (1994), are useful for obtainingthe best orientations of the filter elements for a required transferfunction, and each reference cited is incorporated herein. Thesesynthesis techniques rely on the fact that each element in the filterprovides one cosine Fourier component of the desired transmissionfunction. Such synthesis techniques are useful in situations where theactive element in the filter operates in the 0 to pi/2 range.

A birefringent filter based LCD employing electrically controlledbirefringence (ECB) type cells, which fall into the category ofoptically simple active elements, was disclosed in the above-referencedSaeed et al. publication. The ECB type cells, however, require a ratherexpensive drive method called active matrix addressing, which involvesthe use of a transistor at each pixel edge to provide a switchingoperation.

A less expensive drive method is passive matrix addressing. A passivematrix addressing design could make use of the electro-opticalcharacteristics of the LC mode and could drive displays without the useof transistors, and, as a result, the display would be cheaper andeasier to manufacture. However, the requirement of a sharpelectro-optical curve means that LC modes such as the ECB cannot be usedin conjunction with passive matrix addressing. Thus, there exists a needin the art for a birefringent filter-based color generation scheme for apassive matrix display device.

In the design of a single stage of a color filter, the incident lightcan be considered to have two spectral components: a controlled spectralcomponent and an un-controlled spectral component.

One of the required features of the electro-optical element in the colorfilter of Sharp is that the polarization state of the controlledspectral component of the light is effected by the state of theelectro-optical element, but the polarization state of the uncontrolledcomponent is not.

Sharp used an untwisted liquid crystal device for which it is well knownthat if light is linearly polarized with a particular state (at 45° tothe projection of the optic axis on the plane of the device), that thepolarization state of the light will be affected by a voltage applied tothe device; but if light is polarized parallel to the projection of theoptic axis, then the polarization state will not be affected by avoltage applied to the device.

However, untwisted nematic devices typically do not show thatappropriate voltage versus polarization state change that is requiredfor a multiplexed passive matrix display, and it is not clear, fromSharp, that it could be possible to use a nematic material of twistedstructure. In fact, it is well known that twisted devices do not havethe characteristic that a state of linearly polarized light with a widewavelength spectral band can be unaffected by the state of the device.Therefore, the problem of designing a filter of the type discussed bySharp, but having a twisted, multiplexable, liquid crystal device, isthe focus of this invention.

SUMMARY OF THE INVENTION

In general, the present invention provides a color liquid crystaldisplay of passive matrix design that includes a polarizer, an analyzerhaving a transmission axis, and at least one light generation stagepositioned between the polarizer and analyzer for transmitting light ofcontrolled wavelengths to the analyzer. This light generation stageincludes a first retarder stack that rotates controlled wavelengths oflight to a particular polarization state, a second retarder stackinverted and rotated by 90 degrees with respect to said first retarderstack, and a passive matrix addressed optical element including a firsttwisted liquid crystal cell and a compensator, the element positionedbetween the first and second retarder stacks, wherein the compensatorideally cancels the optical effect of the first twisted liquid crystalcell when the first twisted liquid crystal cell is in a first state, andwherein the compensator does not cancel the optical effect of the firsttwisted liquid crystal cell when the first twisted liquid crystal cellis not in the first state. The retardation and orientation of the firstretarder stack, the second retarder stack, and the first twisted liquidcrystal cell are optimized so as to provide a color generation stagethat, according to the state that the first twisted liquid crystal cellis in, selectively causes the controlled wavelengths of light to bepolarized along the transmission axis of the analyzer, while noteffecting the polarization state of uncontrolled wavelengths.

In particular embodiments, the compensator is selected from the groupconsisting of (a) a second twisted liquid crystal cell, fixed in one ofthe states of said first twisted liquid crystal cell and having anopposite twist sense, and (b) a birefringent film composite constructedto cancel the optical effect of said first twisted liquid crystal cellin one of its states.

A specific embodiment of the present invention provides a full colorliquid crystal display of passive matrix design that includes apolarizer, a red light generation stage, a green light generation stage,a blue light generation stage, and an analyzer having a transmissionaxis. The red light generation stage includes a first red stage retarderstack that rotates red wavelengths of light to a particular polarizationstate, a second red stage retarder stack inverted and rotated by 90°with respect to the first red stage retarder stack, and a red stagepassive matrix addressed optical element positioned between the firstand second red stage retarder stacks, wherein the retardation andorientation of each of the first red stage retarder stack, the secondred stage retarder stack, and the red stage passive matrix addressedoptical element are optimized so as to provide a red light generationstage that places red light wavelengths substantially along thetransmission axis of the analyzer while having no effect on the otherwavelengths of light passing therethrough. The green light generationstage includes a first green stage retarder stack that rotates greenwavelengths of light to a particular polarization state, a second greenstage retarder stack inverted and rotated by 90° with respect to thefirst green stage retarder stack, and a green stage passive matrixaddressed optical element positioned between the first and second greenstage retarder stacks, wherein the retardation and orientation of eachof the first green stage retarder stack, the second green stage retarderstack, and the green stage passive matrix addressed optical element areoptimized so as to provide a green light generation stage that placesgreen light wavelengths substantially along the transmission axis of theanalyzer while having no effect on the other wavelengths of lightpassing therethrough. The blue light generation stage includes a firstblue stage retarder stack that rotates green wavelengths of light to aparticular polarization state, a second blue stage retarder stackinverted and rotated by 90° with respect to the first blue stageretarder stack, and a blue stage passive matrix addressed opticalelement positioned between the first and second blue stage retarderstacks, wherein the retardation and orientation of each of the firstblue stage retarder stack, the second blue stage retarder stack, and theblue stage passive matrix addressed optical element are optimized so asto provide a blue light generation stage that places blue lightwavelengths substantially along the transmission axis of the analyzerwhile having no effect on the other wavelengths of light passingtherethrough In this embodiment, polarized light is input to one of thered, green, or blue light generation stages, and passes through each ofthe stages before being input to the analyzer. Because each stage placesits own primary color (red, blue, or green) along the transmission axisof the analyzer, while having no effect on the other wavelengths oflight passing therethrough, virtually any color can be emitted from theanalyzer.

These and other objects of the present invention, as well as theadvantages thereof over existing prior art forms, which will becomeapparent from the description to follow, are accomplished by theimprovements hereinafter described and claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

For a complete understanding of the objects, techniques and structure ofthe invention, reference should be made to the following detaileddescription and accompanying drawings, wherein:

FIG. 1 is a representation of a prior art folded Solc filter baseddesign having four retarders split by an active liquid crystal cell withuniform optic axis orientation of 90°, wherein the arrows inside theretarders and liquid crystal cell show the orientation of the optic axisof the retarders and liquid crystal, and the arrows within the circlesshow the polarization factor of the desired wavelength;

FIG. 2 depicts a prior art full color liquid crystal display of astacked filter design, employing active matrix addressed opticalelements 4B, 4G, 4R, in three color generation stages 1B, 1G, 1R;

FIG. 3 is a representative diagram of polarized light passing throughthree different stacks of filters (stacks (A), (B), (C)), wherein eachstack contains three filters individually tuned for blue, green, and redwavelengths;

FIG. 4 depicts a full color liquid crystal display according to thisinvention, employing passive matrix addressed optical elements;

FIG. 5 is a graph representing the calculated director tilt and twistprofiles at Vs and Vns for the STN liquid crystal employed according toTable 1;

FIG. 6 shows the spectral response of an optimized stack according tothe present invention;

FIG. 7 shows the CIE 1931 coordinates for the optimized stack ascompared to the NTSC gamut;

FIG. 8 shows a white state spectra for two different filter positions,GBR and BGR, in the stack;

PREFERRED EMBODIMENT FOR CARRYING OUT THE INVENTION

As mentioned, and discussed with respect to FIGS. 1-3, the prior artteaches that the generation of color in a display device employingpolarization interference filters requires a polarizer, a prefilterretarder stack, an active matrix addressed optical element, such as aliquid crystal cell with transistors disposed at each pixel location, apost filter retarder stack, and an analyzer. Similar elements areemployed in the present invention; however, whereas the optical elementof the prior art requires the active matrix addressing drive method, thepresent invention operates through passive matrix addressing byemploying a passive matrix addressing optical element.

Thus, FIG. 4 depicts a full color liquid crystal display of passivematrix design, designated generally by the numeral 100. Similar to FIG.2, display 100 includes a color generation stage for each primary color,blue, green, and red, designated as 10B, 10G, and 10R, respectively.Each stage 10 includes first retarder stack 12 and second retarder stack13, which is inverted and rotated by 90° with respect to first retarderstack 12, with distinctions made between each stage and its elements byemploying the letters B, G, and R to indicate blue, green, and redstages. Whereas the prior art employed active matrix addressed opticalelements, the present invention employs passive matrix addressed opticalelements 14. Particularly, the electrically controlled birefringence(ECB) cells employed in the prior art are herein replaced with a passivematrix addressed optical element including a twisted liquid crystal cell18 and a compensator 19. A polarizer and analyzer are also employed atopposite ends of the stacked stages 10, and are respectively representedby arrow 16 and dot 17, which indicate the transmission axes of theseelements. It will be appreciated that the present invention might bepractice with either orthogonal or parallel polarizer/analyzerconfigurations, and, while the orthogonal configuration produces blue,green, and red spectras, as in FIG. 4, the parallel configuration wouldsimilarly produce yellow, magenta, and cyan spectras. In either case,the stacked configuration can produce any desired color. In the presentinvention, the orientation and retardation of each retarder and thepassive matrix addressed optical element are reworked such that eachlight generating stage places desired wavelengths of light along thetransmission axis of the analyzer, while having no effect on the otherwavelengths of light passing therethrough.

The twisted LC cell has a relatively complex optical characteristic dueto its highly twisted structure. It no longer acts as a waveguide, andits output polarization states are generally never linear in nature. Asa result, the twisted LC cell cannot be substituted for the ECB cell inthe filter designs of the prior art. Also, the fact that twisted LCcells have to be driven at two voltages, neither of which has anisotropic output, means that they cannot be used in the filter designedfor an ECB. By having no isotropic output it is understood that nomatter what orientation linearly input light has with respect to thetwisted LC cells optic axis, the twisted LC cell will always have aneffect on it. In a twisted LC cell, it is impossible to achieve theisotropic condition required to pass white light through unaffected,because the cell, in both voltage cases, is never at zero retardation.This is unlike the ECB cell case where light linearly polarized at 0° orpi/2 with respect to the optic axis passes through unchanged, and is afundamental requirement for the filter design.

In the present invention, employing liquid crystals of twistedstructure, in order to satisfy the requirement that in one state thepassive matrix addressed optical element has a net zero retardation, acompensator is paired with the twisted LC to form the passive matrixaddressed optical element. The twisted liquid crystal cell is a drivenelement to which differing voltages are applied to achieve a desiredoptical state, while the compensator is an inactive element selectedfrom the group consisting of (a) a second twisted liquid crystal cell,fixed in one of the states of the first twisted liquid crystal cell andhaving an opposite twist sense, and (b) a birefringent film compositeconstructed to cancel the optical effect of the first twisted liquidcrystal cell in one of its states.

When the compensator is selected from a second twisted LC cell and thefirst twisted LC cell is in the same state in which the second twistedLC cell (i.e., the compensator) is fixed, it serves to cancel the effectof the first twisted LC cell on the polarization state of light enteringthe passive matrix addressed optical element. When the first twisted LCcell is in a different state than that in which the compensator isfixed, the compensator does not cancel the effect of the first twistedLC cell on the polarization state of light, and a net optical effect isprovided across the passive matrix addressed optical element.

When the compensator is selected from a birefringence film composite,the film is constructed to mock the characteristics of a secondcompensator twisted LC cell, as above. Thus, the film compensator isconstructed to cancel the optical effect of the first twisted liquidcrystal cell in one of its states. A DSTN cell is disclosed below assatisfying the second twisted LC cell compensator concept, but it shouldbe understood that the computer modeling techniques employed to optimizethe DSTN situation could be applied in optimizing a compensator filmembodiment.

The passive matrix addressed optical element (twisted liquid crystal andcompensator) can be satisfied with a double layer super twisted neumatic(DSTN) cell—in such a case, the compensator is a second twisted LC cell.The DSTN cell uses two STN cells. One cell can be driven, and has eithera left or right handed twist, while the other cell is placedorthogonally with respect to the first, and has the opposite twist. As aresult, when the first cell is in the non-select state, similar to thesecond, the two cancel each other out and appear optically isotropic,thereby satisfying one of the requirements. In the select state, thefirst cell is driven while the second is left as is, and the DSTNappears optically active. In the film-compensated situation, a filmcompensated super twisted nematic cell (FCSTN) could be constructed tosatisfy the necessary twisted LC cell and compensator configuration.

The second requirement of having an orientation at which light inputlinearly polarized passes through unaffected is a lot harder to satisfyconsidering the DSTN cell and the complex optics of its driven andundriven state. This requirement has to be satisfied since only thedesired wavelengths should be rotated, leaving the rest of the spectrauntouched. If the DSTN was to rotate any other part of the spectra,then, in a stacked design, the successive filters would not have 100% ofthe spectra with which to work, and efficiency would be considerablylower and unwanted colors would leak.

In this invention, in order to obtain the best orientations and retardervalues, computer modeling is employed to optimize a stack of colorstages, as in FIG. 4. A four uniaxial retarder (2 pre, 2 post filters)and one passive matrix addressed optical element (DSTN) filter waschosen. The number of retarders used here is for the sake of example andease of manufacturing. Any number of retarders could be used since themore retarders used the better the filter performance. The STN mode waschosen to be a 180° twist, with parameters given in Table 1, anddirector profiles at select and non select voltages shown in FIG. 5. Itshould be noted that the parameters of Table 1 have been chosen here forthe sake of providing an example, and should not serve to limit thepresent invention. Indeed, it is believed that the twist of any twistedliquid crystal cell disclosed herein may have a twist anywhere between90° to 270° and preferably between about 180° to about 220°. It willalso be appreciated that the liquid crystal cells disclosed herein couldbe “bistable” devices. Bistable refers to the ability of the cell to beplaced in a particular state or twist by some application of a physicalproperty, typically an applied voltage, and wherein that state or twistremains indefinitely, after removal of the applied voltage of physicalproperty, until a new voltage or property is applied.

TABLE 1 STN Parameters Used In The Modeling Process Twist 180° Thickness5 microns Pretilt  5° Elastic constants K11 = 10, K22 = 7, K33 = 18Dielectric constants Eparr = 15, Eperp = 10 Vs, Vns 2.44, 2.10 d/p 0.5

The extended Jones matrix method was used to obtain the spectral outputof the stack. A. Lien: Liquid Crystal 22 (1997) No. 2, 171. Even thoughobtaining the spectral response of a single configuration of thecomplete stack is a relatively fast operation on a computer, the sheernumber of possible orientations of all the elements in the stack makesit impossible to model the entire R, G, and B filter stack put together.As a result, a single color generation stage was modeled between crossedpolarizers (i.e. polarizer and analyzer). The retardations (R1, R2) andorientations (O1, O2) of the first two retarders (pre filter) in thestack were allowed to vary as well as the retardation of the LC (R3)used in the DSTN and the DSTN's orientation (O3). The last two retardersin the stack used the same retardances as the first two, except thatthey were inverted (R2,R1) as well as orthogonal (R2±pi/2,R1±pi/2) asrequired by the design. The orientation of the retarders was changedfrom −90° to 90° in steps of 15°, and their retardations changed from 20nm to 200 nm in steps of 20 nm. The DSTN angle was changed from −90° to90° in steps of 15°, and its retardation from 100 nm to 380 nm in stepsof 20 nm. For each combination of the above, transmission values werecalculated at four different wavelengths. For example, for the bluestack, light transmission is calculated at 430, 550, and 650 nm. Inorder for the blue filter to be optimized, the characteristics of theelements were written to file if the normalized transmission at 430 nmwas greater than 0.5, at 550 nm was less than 0.125, and at 650 nm wasless than 0.05. Only three points were used for the calculation in lightof the time it takes for each calculation, based upon the sheer numberof different combinations of retarder orientations and the computerresources available. The same was done for the green and red filter casewith properly chosen wavelength points and threshold values. When thetransmission for each filter satisfied the given thresholds, theretarder and DSTN characteristics were saved.

The outputs for the R, G, and B filter were then further refined. Thiswas done by reading in the saved files obtained from the step above andmoving the retarder around the given value, for example, plus or minus5°. In this case the DSTN retardation was varied ±10 nm and its angle±5° around the given value. Only the angular value of the retarderchanged by the ±5°. For each possible combination, the transmission, atwavelengths similar to the above case, were calculated and compared tothe same threshold. If the new orientations satisfied the threshold,they were written to the file. In this way a finer orientation wasobtained around values that were known to work. This significantlyreduced the computation time as compared to a situation in which theoriginal search is done with the finer angular resolution.

In the next step, each of the configurations from above were read fromfile, and, for each configuration, the transmission at differentwavelengths was calculated. In the case of the blue filter, for example,550 nm, 600 nm, 650 nm, and 700 nm wavelength values were used. This wasdone to eliminate configurations that have significant leaks at thosewavelengths, signifying that more than one maxima or a large sidelobeexists in the transmission function. Such a process will eliminate anumber of configurations that are dissimilar to the desired transmissionfunction.

The configurations that did satisfy the thresholds were written to thefile for each filter. Next, these configurations were read from the fileand further refined. In this step, only the retarder angles wererefined, by allowing the angles to float ±5° from the ones read in fromfile and varied in steps of 0.25°. This was done to obtain the Solcdefined angles for a filter using two retarders. The same thresholdconditions as in the step above were applied and the configurationssatisfying the threshold were written to file.

In order to rank the configurations obtained above for each filter, theCIE 1931 color coordinate for each configuration in the file wascalculated. For each coordinate, its position with respect to the NTSCcolor coordinates was calculated. The configurations were then rankedfrom closest to furthest from the NTSC color coordinate for each filter.However, the proximity to the CIE color coordinate alone does notsignify that the configuration is the best, since its overall luminancecan be low. As a result, the first 100 configurations were analyzed, andthe one with the brightest output was chosen.

Using this method, the orientation and retardation of the elementsmaking up the three different color filters were chosen. By ensuringhigh peak brightness at the desired wavelength along with sharptransition slopes and minimum overlap, the filter was effectivelyrotating the desired wavelength by pi/2, while leaving the rest of thespectra unchanged, thereby fulfilling the third condition defined above.The spectral response of the three filters put together to form acomplete stack is shown, in FIG. 6, along with the CIE coordinates, inFIG. 7. The lower transmission of the blue and green state is due topolarizer characteristics at those wavelengths, and is not due to thefilter design. The filter is transmitting the polarizer's maximumtransmission at those wavelengths. The stack details are shown in Table2 below.

TABLE 2 Configuration for DSTN based filter providing best colorperformance GREEN BLUE RED Retar- Retar- Retar- dation Angle dationAngle dation Angle DSTN 1950 nm  5° 750 nm −40° 1150 nm  40° R1  700 nm−40° 600 nm  14.5°  500 nm −10.5° R2  500 nm −70° 800 nm −14°  800 nm 13.5° R3  500 nm  20° 800 nm  76°  800 nm −76.5° R4  700 nm  50° 600 nm−75.5°  500 nm  79.5°

Since the optimization was done using single color filters, and somecolors, such as green, have a inherently wider passband, the position ofthat filter in the overall stack has an effect on the white state. Thewhite state is where all three filters are rotating their desiredwavelengths. The stack was modeled by placing the optimized filters invarious positions in the stack, i.e., BGR, GBR, GRB, RBG etc., with theleftmost stack positioned immediately adjacent the polarizer, and therightmost stack positioned immediately adjacent the analyzer. From this,it was ascertained what positioning is best. In the example beingcarried out, the GBR position was the best. In all the differentpositions the spectral response of the single color state and blackstate were similar. The white state was the only one that changed due todifferent positioning. This was due to the green cells relativelybroader passband rotating certain parts of the blue and red spectra. Thespectral response of the white state only is shown for the BGR and GBRstates. in FIG. 8.

In the present example, the GBR state was chosen as best because it hasa dip in the blue part of the spectrum (as compared to the BGRorientation which has a dip in the red). Since most backlights are redlacking, it was decided to use the GBR orientation so as to maximize redthroughput in the white state. The BGR and GBR white points are shown inCIE diagram FIG. 6 and the lowered red transmission of the BGRconfiguration is visible in terms of a shift in the white point to theleft. The GBR white point is more centered.

Thus, disclosed above is the optimization of a full color liquid crystaldisplay of passive matrix design, wherein the passive matrix addressedoptical element employed is a first twisted LC cell and a second twistedLC cell driven at the same non-select voltage as the first twisted LCcell and placed orthogonally to the first twisted LC cell. The secondtwisted LC cell is considered a compensator. When the compensator ischosen as a birefringence film composite, the compensator would beconstructed, through known techniques, to mock the characteristics of asecond twisted LC cell compensator, were one to be employed. Thus, in abirefringence film compensator, the film compensator is constructed tobe a substantial mirror image of a twisted LC cell driven at the samenon select voltage as the first twisted LC cell and placed orthogonallyto the first twisted LC cell. Computer modeling would be employed, asabove, to optimize all parameters of a full color liquid crystal displayemploying a passive matrix addressed optical element of a first twistedLC cell and a birefringence film composite compensator.

Therefore, after an exhaustive hunt, using computer modeling, it hasbeen shown that a PIF filter, with characteristics similar to the deviceof Sharp, can be obtained using a liquid crystal device with a twistedstructure. The device of this invention are characterized, in part, byusing a compensator that cancels the effect of the twisted liquidcrystal on the polarization state of light, when the device is in adefined state; and that allows light of the uncontrolled spectralcomponent to be unaffected by the state of the electro-optical device.

In light of the foregoing, it should thus be evident that the process ofthe present invention, providing a birefringent filter-based colorgeneration scheme for a passive matrix display device, substantiallyimproves the art. While, in accordance with the patent statutes, onlythe preferred embodiments of the present invention have been describedin detail hereinabove, the present invention is not to be limitedthereto or thereby. Rather, the scope of the invention shall include allmodifications and variations that fall within the scope of the attachedclaims.

1. A color liquid crystal display of passive matrix design comprising: apolarizer; an analyzer having a transmission axis; and at least onelight generation stage positioned between said polarizer and saidanalyzer for transmitting light of controlled wavelengths to theanalyzer, the light generation stage including: a first retarder stackthat rotates controlled wavelengths of light to a particularpolarization state, a second retarder stack inverted and rotated byabout 90 degrees with respect to said first retarder stack, and apassive matrix addressed optical element including a first twistedliquid crystal cell and a compensator, said element positioned betweensaid first and second retarder stacks, said compensator being selectedfrom the group consisting of (a) a second twisted liquid crystal cell,fixed in one of the states of said first twisted liquid crystal cell andhaving an opposite twist sense, and (b) a birefringent film compositeconstructed to cancel the optical effect of said first twisted liquidcrystal cell in one of its states, wherein said compensator ideallycancels the optical effect of said first twisted liquid crystal cellwhen said first twisted liquid crystal cell is in a first state, andwherein said compensator does not cancel the optical effect of saidfirst twisted liquid crystal cell when said first twisted liquid crystalcell is not in said first state, and the retardation and orientation ofsaid first retarder stack, said second retarder stack, and said firsttwisted liquid crystal cell are optimized so as to provide a colorgeneration stage that, according to the state that said first twistedliquid crystal cell is in, selectively causes said controlledwavelengths of light to be polarized along the transmission axis of saidanalyzer, while not affecting the polarization state of uncontrolledwavelengths.
 2. The color liquid crystal display of claim 1, whereinsaid passive matrix addressed optical element is a double layer supertwisted nematic cell.
 3. The color liquid crystal display of claim 1,wherein said first twisted liquid crystal cell is chosen to have a twistanywhere between about 180° to about 220°.
 4. The color liquid crystaldisplay of claim 1, wherein said at least one light generation stage isa stage selected from the group consisting of a red light generationstage, a green light generation stage, and a blue light generationstage.
 5. The color liquid crystal display of claim 4, wherein a redlight generation stage, a green light generation stage, and a blue lightgeneration stage, each said stage controlling a corresponding range ofwavelengths of light, are positioned adjacent one another between saidpolarizer and said analyzer.
 6. The color liquid crystal display ofclaim 4, wherein a green light generation stage is positionedimmediately adjacent said polarizer; a blue light generation stage ispositioned adjacent said green light generation stage, opposite saidpolarizer; and a red light generation stage is positioned between saidblue light generation stage and said analyzer.
 7. The color liquidcrystal display of claim 4, wherein a blue light generation stage ispositioned immediately adjacent said polarizer; a green light generationstage is positioned adjacent said blue light generation stage, oppositesaid polarizer; and a red light generation stage is positioned betweensaid green light generation stage and said analyzer.
 8. The color liquidcrystal display of claim 1, wherein said compensator is a second twistedliquid crystal cell, fixed in one of the states of said first twistedliquid crystal cell and having an opposite twist sense.
 9. The colorliquid crystal display of claim 2, wherein said compensator is abirefringent film composite constructed to mock the opticalcharacteristics of a second twisted liquid crystal cell having the samestate as one of the states of the first twisted liquid crystal cell butwith an opposite twist sense.
 10. A full color liquid crystal display ofpassive matrix design comprising: a polarizer; an analyzer having atransmission axis; a red light generation stage including a first redstage retarder stack that rotates red wavelengths of light to aparticular polarization state, a second red stage retarder stackinverted and rotated by 90° with respect to the first red stage retarderstack, and a red stage passive matrix addressed optical elementpositioned between said first and second red stage retarder stacks,wherein the retardation and orientation of each of said first red stageretarder stack, said second red stage retarder stack, and said red stagepassive matrix addressed optical element are optimized so as to providea red light generation stage that places red light wavelengthssubstantially along the transmission axis of said analyzer while havingno effect on the other wavelengths of light passing therethrough; agreen light generation stage including a first green stage retarderstack that rotates green wavelengths of light to a particularpolarization state, a second green stage retarder stack inverted androtated by 90° with respect to said first green stage retarder stack,and a green stage passive matrix addressed optical element positionedbetween said first and second green stage retarder stacks, wherein theretardation and orientation of each of said first green stage retarderstack, said second green stage retarder stack, and said green stagepassive matrix addressed optical element are optimized so as to providea green light generation stage that places green light wavelengthssubstantially along the transmission axis of the analyzer while havingno effect on the other wavelengths of light passing therethrough; and ablue light generation stage including a first blue stage retarder stackthat rotates blue wavelengths of light to a particular polarizationstate, a second blue stage retarder stack inverted and rotated by 90°with respect to said first blue stage retarder stack, and a blue stagepassive matrix addressed optical element positioned between said firstand second blue stage retarder stacks, wherein the retardation andorientation of each of said first blue stage retarder stack, said secondblue stage retarder stack, and said blue stage passive matrix addressedoptical element are optimized so as to provide a blue light generationstage that places blue light wavelengths substantially along thetransmission axis of the analyzer while having no effect on the otherwavelengths of light passing therethrough; wherein each of said bluestage passive matrix addressed optical element, said green stage passivematrix addressed optical element, and said red stage passive matrixoptical element include a first twisted liquid crystal cell and acompensator.
 11. The full color liquid crystal display according toclaim 10, wherein said compensator in each of said blue stage passivematrix addressed optical element, said green stage passive matrixaddressed optical element, and said red stage passive matrix opticalelement is selected from the group consisting of (a) a second twistedliquid crystal cell fixed in one of the states of said first twistedliquid crystal cell of said respective stage and having an oppositetwist sense and (b) a birefringent film composite constructed to cancelthe optical effect of said first twisted liquid crystal cell of saidrespective stage in one of its states.
 12. The full color liquid crystaldisplay of claim 10, wherein each of the red stage, green stage, andblue stage passive matrix addressed optical elements is a double layersuper twisted nematic cell.
 13. The full color liquid crystal display ofclaim 10, wherein each said first twisted liquid crystal cell is chosento have a twist anywhere between about 180° to about 220°.